An electric car is known as a vehicle in which a motor generator is utilized as a power source. When the electric car is moving or when it is accelerating, the electric power stored in a storage battery (hereinafter, occasionally referred to as a “battery”) is supplied to the motor generator that works as a motor. When the car is braked, the regenerated electric power obtained by operating the motor generator as a generator is stored in the battery.
A hybrid car is also known as a vehicle in which a motor generator is utilized as a power source. In the hybrid car, the regenerated electric power obtained by operating the motor generator as a generator at the time of braking is stored in the battery. When the car is moving or when it is accelerating, the electric power stored in the battery is supplied to the motor generator that works as a motor, and an internal combustion engine serving as another power source is assisted by the driving force created by the motor generator, so that fuel consumption is improved. Such a hybrid car is disclosed, for example, in Unexamined Japanese Patent Publication No. 2005-151633.
However, for example, if regenerative braking is used on an electric or hybrid car (hereinafter, occasionally referred to as “electric car or the like”) for a prolonged time or with frequency on a downhill or the like, there is the possibility of a battery overcharge. It is then necessary to switch the regenerative braking to mechanical brake or use the both, and accordingly, the kinetic energy that is essentially recoverable as regenerative energy is lost because of the mechanical brake. It is therefore difficult to say that the kinetic energy generated during vehicle deceleration can be fully recovered.
In a situation where an overcharge occurs, a rise in battery temperature increases electric resistance (hereinafter, occasionally referred to as “resistance”) of electrodes or the like in the battery, and thereby power loss in the battery is increased. In addition, the heavy usage of the mechanical brake quickly wears a brake shoe and the like.
One way to solve these issues is to increase the charging capacity of the battery. On the other hand, if a lot of batteries are installed in an electric car or the like, there causes problems, such as a reduction in a passenger's space in the vehicle and an increase in weight of the vehicle body. When the vehicle is a hybrid car, the concurrent use of an engine brake is possible. However, as seen in the case where a mechanical brake is concurrently used, there still remains the problem that the kinetic energy generated during vehicle deceleration cannot be fully recovered.